PET is a thermoplastic polymer resin of the polyester family and is commonly used in beverage, food and other liquid containers. To form a container such as a bottle, one or two-step molding methods may be used. For example, in a two-step method a preform is injection-molded. Next, on a second machine, the preform is inflated into its final shape using stretch blow molding.
PET is commonly supplied in the form of PET chips or flakes. The PET chip material is brought to a high temperature to be molded. An extruder provides the melted PET material to a preform mold.
However, in the solid form such as PET chips PET has a high hygroscopic behavior. PET chips absorb moisture from the atmosphere until equilibrium is achieved. The PET chips supplied for processing are water-saturated, and may contain up to 0.6% water by weight. During plasticizing in the extruder, the presence of moisture breaks polymer chains. Any water present at this stage rapidly hydrolyses the polymer, thereby reducing its molecular weight and damaging its physical properties.
More particularly, breaking polymer chains of PET results in a drop in IV (intrinsic viscosity) of the PET. The intrinsic viscosity is commonly used to characterize PET material. It is a measure of the polymer molecular weight. The longer the polymer chains and the more entanglements between chains the higher the viscosity. A drop in IV may result in defects in the molded preform or container, such as growth of bubbles, streaks, or a hazy appearance. In addition, degradation through hydrolysis of PET may cause acetaldehyde, benzene, and/or formaldehyde generation. Typically, acetaldehyde can cause an off-taste in bottled water: that is why its generation must be avoided.
According to commonly accepted good practice in PET molding, PET has to be dried to very low moisture content before molding. Indeed, the moisture content is reduced to a maximum of 0.005% (50 ppm) by weight, and preferably about 0.003% (30 ppm).
Drying is performed in a PET dryer. Typical PET dryers use a closed loop in which hot and dry air circulates. PET chips or flakes are placed in a hopper, and hot dry air flows onto the material, from which it absorbs moisture. The air is next conducted into a drying unit where it is cooled and de-humidified. De-humidifying is carried out using a desiccant bed (generally in a desiccation tower) that must be periodically regenerated. De-humidified air is next heated again before being passed through the hopper again.
Three important parameters of the drying air are air temperature, air flow and air dew point. The residence time of the PET in the hopper (drying time) is a fourth important parameter in PET drying. Indeed, because of the hygroscopic behavior of PET, moisture is present inside the PET chips. Usually a residence time of about six hours at 180° C. and a dew point of −60° C. is necessary to bring the material to less than 50 ppm of water in weight.
In addition to the above detailed hydrolytic degradation, the PET may be degraded through oxidative degradation and thermal degradation. The drying temperature generally used is around 170° C., and should not exceed 190° C., also depending whether recycled PET (rPET) chips or flakes are used. The drying temperature is the temperature of air used for drying the PET, and also corresponds to the temperature of the PET at the hopper exit.
To obtain the required PET dryness, the air dew point (representing air dryness) should be around −60° C.
The drying parameters of PET chips must be carefully controlled according to strict and well established rules.
PET manufacturers sell PET in the form of chips or flakes. Customers' (e.g. container manufacturers') demand for PET having a high intrinsic viscosity (IV) has grown in the past years. PET having a high IV (hereafter “high-IV PET”) has better mechanical properties, and bottles used to contain sparkling beverages have to be made of high-IV PET. Typical high-IV PET which is available on the market has an IV of 0.80 dL/g or above.
PET having a lower IV, typically between 0.72 dL/g and 0.76 dL/g, is sufficient for still beverage bottles, such as still water bottles. However, such PET which was formerly widely available on the market is now rare and expensive. This is because the demand for high IV PET is much greater than the demand for low-IV PET, and also because a significant quantity of recycled PET, which has a high IV, is commonly incorporated into the material of the PET flakes. However, low IV PET has some advantages over high IV PET. It is easier to inject for molding. It allows a better, more homogeneous, distribution in the mold in which it is injected. It allows manufacture of preforms having a wall thickness of less than 2 mm, which difficult if not is impossible with high-IV PET. A preform having a small wall thickness is needed to obtain a lightweight thin-walled bottle.
In addition, some industrial equipment used to inject preforms (or, more generally, to inject PET into a mold) is not compatible with high-IV PET. Indeed, injecting high IV PET requires a higher injection pressure, and/or a higher injection temperature than injecting a PET having a lower IV. Expensive low-IV PET has to be used in these industrial machines, or else they must be adapted to make possible the use of high IV-PET.